This invention relates to loudspeakers which produce sound in response to an audio signal. In particular, this invention relates to a loudspeaker which is capable of handling sustained high power levels without substantial loss of efficiency due to resistance heating.
Most modern loudspeakers employ a diaphragm which is vibrated by an electromechanical drive. In a conventional dynamic loudspeaker, the drive includes a voice coil which is connected to the audio amplifier. The voice coil is suspended between the pole pieces of a permanent magnet and is connected to a speaker cone. It is also known, for example, in U.S. Pat. No. 4,564,727, to drive a speaker diaphragm with a commutated motor, especially for the purpose of producing low frequency sound.
From the above, it may be seen that most loudspeaker drivers employ coils or windings of conductive material, which carry alternating DC current for their operation. In operation, the resistance of the conductive material causes production of heat and an increased temperature in the voice coil or winding. Because the DC resistance comprises a major portion of a driver's impedance, most of the input power is converted into heat rather than sound. As a result, the ultimate power handling capacity of a driver is strictly limited by the ability of the device to tolerate heat. This, in turn, may be determined by factors such as melting points of the components and heat capacity of the adhesive used to construct the voice coil.
The problems of heat generation is further compounded by temperature induced resistance, commonly referred to as power compression. The D.C. resistance of copper or aluminum conductors or wires used in the driver increases as the temperature increases. For example, a copper wire voice coil having a room temperature resistance of six ohms will have a resistance of twelve ohms at 230.degree. C., which will occur when power input is increased. Thus, a point is soon reached at which additional power input will be converted mostly into additional heat rather than sound, thereby posing a serious limitation or driver efficiency.
No adequate solution has been forthcoming to deal with or resolve the problems of poor heat capacity or power compression. Any proposed solution has involved trade-offs. For example, it is possible to use heavier and thicker wire in the voice coil to reduce resistance, but this increases the moving mass and decreases efficiency.